1. Field of the Invention
The present invention relates to techniques for transmitting electrical signals. More specifically, the present invention relates to a method and an apparatus for transmitting electrical signals through balanced bundles with a three-dimensional structure, wherein rising and falling transitions are substantially matched, and without doubling the number of signal lines as is done in conventional differential signaling.
2. Related Art
As processor clock speeds continue to increase at an exponential rate, data must be transferred at correspondingly faster rates between computer system components. Computer systems typically use large parallel busses for this purpose.
These large parallel busses typically use either single-ended signaling or differential signaling. Single-ended signaling makes use of a single signal line to carry each bit, along with one or more clock lines to latch the signals.
In contrast, differential signaling uses two signal lines to carry each bit, wherein the value of the bit is indicated by a voltage difference between the two signal lines. Because currents are balanced between power and ground rails, differential signaling reduces power supply noise and solves the problem of where return currents come from. Moreover, differential signaling is less sensitive to ground shifts between sender and receiver because differential signaling relies on voltage differences between pairs of signal lines, instead of relying on an absolute voltage level of a single signal line.
Unfortunately, differential signaling uses twice as many wires as single-ended signaling, which can greatly exacerbate pin limitation problems.
What is needed is a method and apparatus for transferring data between computer system components without the large number of signal lines required by differential signaling, and without the current balance and ground noise problems of single-ended signaling.
One embodiment of the present invention provides a system that transmits a stream of datawords through a bundle of conductors with a three-dimensional structure. Upon receiving a dataword to be transmitted, the system uses an encoding function to encode the dataword into a current codeword in a stream of codewords, wherein the current codeword is less than double the size of the dataword. Next, the system transmits the current codeword to a destination through the bundle of conductors. Note that the encoding function depends on a preceding codeword in the stream of codewords, so that when the preceding codeword changes to the current codeword, rising transitions are substantially matched with falling transitions within the bundle.
In a variation on this embodiment, the encoding function ensures that each conductor with a rising transition matches a corresponding conductor with a falling transition. In a further variation, the encoding function ensures that matching conductors are adjacent to each other within a two-dimensional cross-section of the three-dimensional structure of the bundle. Typically, this cross-section is normal to the direction of signal propagation in the bundle.
In a variation on this embodiment, conductors in the bundle form a hexagonal mesh, wherein each conductor in the interior of the bundle is adjacent to six neighboring conductors.
In a variation on this embodiment, the encoding function ensures that the current codeword is different from the preceding codeword, whereby no clock signal needs to be forwarded with the current codeword.
In a variation on this embodiment, the bundle of conductors is adjacent to other bundles of conductors in a larger three-dimensional structure. In a further variation, adjacent bundles have staggered clock phases.
One embodiment of the present invention provides a system that decodes a stream of codewords received through a bundle of conductors with a three-dimensional structure. Upon receiving a current codeword in the stream of codewords, the system uses a decoding function to decode the current codeword to restore a corresponding dataword, wherein the current codeword is less than double the size of the dataword. This decoding function reverses a corresponding encoding function, wherein the encoding function for the dataword depends on a preceding codeword in the stream of codewords, so that when the preceding codeword changes to the current codeword, rising transitions are substantially matched with falling transitions within the bundle.